1. Field of the Invention
This invention generally relates to a radio-conductive material which is a photoconductive material sensitive to radiations, a method of manufacturing such a radio-conductive material, a solid sensor which is provided with radio-conductive material layer on which image information can be recorded as a pattern of electrostatic charges (an electrostatic latent image) formed upon exposure to radiations, a method of manufacturing a film of such a radio-conductive material, and a radiation image read-out apparatus using such a solid sensor.
2. Description of the Related Art
In order to reduce irradiation dose to the patient's and/or to improve diagnostic performance of the radiation image in a medical radiography, there have been proposed various systems in which a photoconductive body sensitive to a radiation (e.g., a-Se (amorphous selenium) plate) is used as an electrostatic recording medium, and an electrostatic latent image formed on the photoconductive body upon exposure to a radiation is read out by a laser beam or a number of electrodes. For example, see U.S. Pat. Nos. 4,176,275, 5,268,569, 5,354,982, and 4,535,468, “23027 Method and Device for recording and transducing an electromagnetic energy pattern”; Research Disclosure June 1983, Japanese Unexamined Patent Publication No. 9(1997)-5906, U.S. Pat. No. 4,961,209, and “X-ray imaging using amorphous selenium”; Med Phys. 22 (12).
These systems are higher in resolution than known fluoroscopy using a TV pickup tube and less in irradiation dose to the patients than xeroradiography.
The radio-conductive material for forming the photoconductive body of the systems described above should be high in resistance in the dark, withstand a high electric field (e.g., 105 to 106 V cm−1), be high in radiation absorbance and be able to establish a high electric charge. Further the radio-conductive material should be able to form a film so that electric charges can move in the film without being trapped.
However the selenium radio-conductive materials which have been generally used in the conventional systems are difficult to form a film thin enough to prevent electric charge trapping and are not satisfactory in radiation absorbance though they are excellent is durability and able to establish high electric charges. Further since selenium is designated as poison by law, it is preferred that the radio-conductive material does not contain selenium.
As substitute radio-conductive material for selenium, inorganic/organic composite material formed of organic material and VB–VIB, VB–VIIB, IIB–VIB, IIB–VB, IIIB–VB, IIIB–VIB, IB–VIB or IVB–VIIB inorganic material is disclosed in U.S. Pat. No. 5,556,716. For example, composite radio-conductive materials formed of inorganic material selected from the group consisting of BiI3, PbI4, PbI3 and Bi2S3 and organic material selected from the group consisting of nylon 11, PVK (N-polyvinyl carbazole) and PMMA (polymethyl methacrylate) are disclosed in the patent.
That BiI3/nylon 11 (50%/50% by weight) is a radio-conductive material which exhibits good radio-conductiveproperties is described in “Science, 273(1966), 632”.
The prior arts described above make it feasible to employ heavy element compounds, such as BiI3 which have been said to be not suitable as radio-conductive material since they are difficult to form a high quality film in a large area, are large in dark current and accordingly are not able to withstand a high electric field though being excellent in radiation absorbance, as radio-conductive material by forming a composite of such heavy element compounds with organic material (high-molecular material) which is relatively easy to form a high quality thin film, is small in dark current and is excellent in dielectric properties.
However the inorganic/organic composite materials are disadvantageous in that dispersion of the inorganic material (inorganic fine particles) in the organic material is apt to be deteriorated. That is, since the inorganic/organic radio-conductive materials are generally manufactured by so-called “melt deposition process” in which organic material 81 is melted on a substrate 83 (FIG. 8) heated by a hot plate 82, inorganic particles are added to the molten organic material 81, and the mixture is stirred by, for instance, a spatula to form a film, it is difficult to uniformly disperse the inorganic particles in the organic material. When dispersion of the inorganic particles is not satisfactory, agglomerates of the inorganic particles are left in the formed thin film. The agglomerates deteriorate radio-conductive properties and durability to high electric fields, and can cause electric charge trapping.
Further, since, in the inorganic/organic composite materials, the radio-conductive inorganic material is “diluted” by the organic material, the inorganic material content should be high in order to ensure a satisfactory radiation absorbance. For example, in the case of BiI3/nylon 11, the BiI3 content should be not smaller than 65 wt % in order to obtain a radiation absorbance equivalent to the conventional radio-conductive material of selenium under normal diagnostic radiographic conditions. In known combinations of inorganic material and organic material, it is impossible to uniformly disperse the inorganic material in such a high content, and accordingly, the radio-conductive properties of the radio-conductive material obtained cannot be high.
Further, when a thin film such as a radio-conductive layer is formed of the inorganic/organic radio-conductive material, voids can be embedded in the film, which reduces the inorganic material content in the inorganic/organic radio-conductive material and further deteriorates the radio-conductive properties of the radio-conductive material. Further the voids can deteriorate transfer efficiency of electric charges formed upon exposure to radiations.
Further, since the inorganic/organic composite radio-conductive material is in the form of a nano-composite in which the inorganic material is in the form of fine particles of several nanometers to several tens of nanometers, fine unevenness exists on the surface of a solid sensor formed of the inorganic/organic composite radio-conductive material, and accordingly, an electrode formed on the surface of the solid sensor, for instance, deposition of Au cannot microscopically be in close contact with the radio-conductive layer. That is, gaps are microscopically formed between the electrode. Such gaps form charge wells in which electric charges formed in the radio-conductive layer concentrates, which can cause a stray current.